This invention refers to a process for making and soldering electrical connection beads on electrical connection contact surfaces of electronic circuits or components. This invention also refers to a device for implementing the process of the invention.
Making electrical connections of electronic components with metal beads is a known practice. One of the known techniques consists in making the beads and then putting them on the components. These beads are manufactured separately to the desired diameter using an alloy such as, for example, a tin or lead alloy. They are picked up by a mechanical device and then dipped in flux and finally deposited on the contact surfaces of the component.
The soldering is done in an oven.
This process has the benefit of producing perfectly calibrated beads. However, making these beads is costly. The price is roughly one centime per bead. In addition, the operation is a two-step one: one step to manufacture the beads and another one to install them, thus increasing the cost of the component.
The second known technique consists in making the beads directly by melting solder paste and depositing it by silk-screening on the contact surfaces of the component. This method is highly productive in terms of implementation and very attractive in terms of price, since these beads cost only two fifths as much as those of the foregoing technique. But this technique can only be used for small beads.
The solder paste consists of approximately 50% metal by volume, with the remainder consisting of an organic part or flux necessary to transfer the paste adequately by silk-screening, by giving it the appropriate rheology properties. After reflow, only the metal portion of the volume deposited in each hole of the stencil participates in the making of the bead by coalescence. The desired dimensions of the beads and the pitch between the beads lead to the definition of a maximum hole in the stencil and the stencil""s thickness. At a given hole diameter it will not be possible to exceed a certain thickness value of the stencil; otherwise, the dot of solder paste will not detach completely from the stencil and a portion of the paste will cling to the stencil. After reflow, the bead will be smaller.
With this technique, it will not be possible to achieve suitable bead sizes and, especially, perfect regularity between the beads, because some solder paste will always be dragged on the stencil even if the stencil has appropriate dimensions.
Patent EP 0 753 988 describes a method for shaping solder beads on a substrate. This method consists in using one or several stencils (10) with nozzles (14) into which solder paste is inserted (18). The bottom of the stencil nozzles is plugged with a thin plate (12). The paste filling the nozzles is melted to separate the metal portion, for soldering, from foreign matter (24) which rises to the surface. To avoid the soldering plug which was shaped from falling when removing the plate (12), the upper extremity of the stencil nozzles is covered with another plate (26) and then the stencil is turned around. The plate (12) can then be pulled out. After turning the stencil around again, the stencil nozzles are positioned above the areas to be soldered. By raising the temperatures, the soldering plugs melt and become deposited on the area to be soldered in the shape of a ball.
U.S. Pat. No. 5,346,118 describes an assembly method by way of soldering of electronic components and a method for shaping soldering beads onto one or several components for their assembly. The soldering beads are made with a stencil with conic or pyramid-shaped nozzles filled with solder paste. This patent provides much data on the shape of the stencil nozzles and the composition of the solder paste, but defines imprecisely the method for producing the beads.
U.S. Pat. No. 5,346,118 describes a procedure for laying down sufficient solder paste on a substrate to form large enough beads by way of reflow. In this patent, the inventor found a means of unmolding the solder paste by way of conic openings. This patent shows the usual known silk-screen process consisting of:
1xe2x80x94Laying out the solder paste with a serigraph onto a substrate;
2xe2x80x94Removing the stencil at ambient temperature and prior to reflow;
3xe2x80x94Do the reflow.
The novelty of this patent is limited to the conical holes facilitating the unmolding, but increasing the difficulty of the filling in, and hence the volumes of deposited paste are not necessarily identical.
U.S. Pat. No. 5,658,827 describes a method for producing soldering beads on a substrate by using a stencil. In this patent, the separation of the stencil and the substrate is conducted after cooling, requiring to produce a second reflow.
If the stencil acting as the mold is removed after solidification of the alloy following the reflow, we find that the alignment of the beads along the xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d axes defining the plane on which they are resting is imperfect. One also encounters variations in the height and shape of the beads that may make the end result fall outside the tolerances of the JEDEC standard.
These excessive height and shape variations and this misalignment, or poor alignment, are due to the mechanical constraints imposed first of all by the flux residues during solidification of the alloy since, at that time, the volume available in the mold is shared between the alloy and the residues. Secondly, even a very slight incorrect centering of the stencil-mold with respect to the tabs of the contact surfaces, which is always possible, is likely to create an imbalance of the bead produced by the reflow. Indeed, the bead will naturally seek a position of equilibrium and, for this reason, will assume a non-spherical position allowing it to balance the surface tensions which, in this case, are being exerted in a non-symmetrical fashion.
These defects can be overcome by performing a second reflow after the initial reflow, solidification, and removal of the stencil-mold. Provided that the residues of the denatured flux have been eliminated, since the beads do not undergo any external stress from the stencil-mold, which is absent at this stage of the process, the beads must theoretically find their equilibrium position and assume perfectly aligned positions and a spherical shape.
In actuality, the second reflow is not itself sufficient, since the flux has been denatured and eliminated by cleaning, and the end result will not be perfect with regard to the alignment of the beads and their spherical shape and/or their height or diameter. Some solder flux must first be added prior to the second reflow.
After that, the denatured flux will have to be removed by another cleaning. For this reason, this process with two reflows appears to be particularly complex.
The invention concerns a method allowing to obtain very uniform balls with an excellent alignment along the xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d axes, defining the plane on which they are laid out. To this end, the process of this invention for making and soldering electrical connection beads on the electrical-connection contact surfaces of electronic components is characterized essentially by the fact that:
it uses solder paste having appropriate rheology properties comprising metal microbeads capable of coalescing and being soldered on the metal contact surface and a binder;
said paste is deposited on the contact surface or surfaces by silk-screening;
the stencil used for this purpose has a thinness, hole dimension, and hole spacing determined according to the pitch of the beads to be made and their desired diameter;
after the holes in the stencil have been filled with a scraper or a spreading tool such as that described in U.S. application Ser. No. 09/646412, another high temperature reflow is performed;
during this reflow, the stencil is kept in place on the component and it is separated from the component during or after the reflow, but prior to solidification of the beads, with the beads still being in the molten state so that the beads, as they are forming, find their equilibrium position on their own;
a cleaning of the denatured flux or binder is done after solidification of the beads.
Since the stencil serving as the mold is removed while the beads are still in the unsolidified liquid state, even if there is poor alignment of the stencil with the circuit at this stage, it does not prevent the beads from wetting the contact surface.
A minimum amount of contact surface placed opposite the molten alloy is enough to hold the molten alloy when the stencil is removed.
When the stencil is removed, the bead or beads are deformed by the stresses they undergo as a result of the friction on the walls of the holes in the stencil as well as the surface energy of these walls. Still, since the stencil used by the invention is not wettable and has a low surface energy, the beads remain adhered to their contact surfaces.
Once the bead is free from the hole in the stencil where it was created, it can assume its final position perfectly in the center of the contact surface and acquire a perfectly spherical shape, given that the molten alloy is no longer subjected to any mechanical action either by the stencil or by the residues of binder or flux since, at this time, they are in the liquid state and their density is considerably lower than that of the molten alloy.
For this reason, these liquid residues do not cause any significant resistance while the alloy is taking on a spherical shape.